1. Field of the invention
This invention relates to optical sensors, more specifically to filters for said sensors.
2. Background of the invention
Optical sensing systems, either in the visible range, or the non-visible, such as infrared, can fall victim to jamming effects. In visible systems, this is normally referred to as veiling. Veiling illuminance is defined as light that is spread across a large portion of the field of view, lessening system effectiveness by lowering scene contrast. Common sources of veiling are sunlight and high-intensity sources. Jamming can also be caused deliberately, such as in defense applications, when an enemy electro-optic source is aimed at the subject system.
A number of sensors currently in use contain fixed stop-band interference filters in slides or wheels for protection from these problems. This approach supplies a limited number of stop-band filter choices and requires operating mechanisms with control circuitry, whether they are mechanical or electromechanical. Activation of these filters takes fractions of seconds, not including operator response time. This fixed-filter approach works well for single-wavelength, non-agile threats. However, fixed-filter techniques are limited against the agile threats expected on the modern battlefield.
Another weakness of the fixed-filter system becomes apparent when considering bandwidth. In the 8 to 12 micron (10.sup.-6 meter, or .mu.m) wavelength range, it is not practical to use filters having a bandwidth less than 1 micron. For an infrared system, such as a Forward-Looking InfraRed (FLIR), the integrated transmission of a filter with a single 1 micron stop-band is no better than 50%. Narrower-band filters tend to be more expensive and have insufficient integrated transmission.
Another area of concern with current technology is the difficulty of automation. The system detector can be used to sense when a filter is needed. However, the sensor cannot detect when the filter is no longer needed, since the filter prevents the sensor from seeing the threat. The sensor could be damaged if the filter is switched out and the threat is still present. The long cycle times of mechanical filter assemblies increase the likelihood of damage, if the threat is still present.